1. Field of the Invention
The present invention relates to a circuit architecture for video applications and in particular to a filter architecture for video signals. The present invention concerns also a filtering method for television signals. Specifically the present invention concerns a filter architecture for high-resolution video applications of the type comprising at least one filtering block having a plurality of digital inputs which receive, through an interface, components of a three-dimensional television signal and some outputs through which to take the result of a filtering operation for reducing the noise associated with the television signal.
2. Discussion of the Related Art
Television receiver image quality improvement is correlated on the one hand with the capability of reducing the noise accompanying the video frequency signal and on the other hand with the reduced signal band amplitude.
Noise is an inherent characteristic of any telecommunications system or television signal transmission network and can arise in many different manners. For example, the signal source can be also a source of noise which is generated in a camera. The electronic circuitry designed for conveyance of the signal adds, in general, noise such as in video recorders. In addition, the signal is sent to the transmitters through a distribution network and broadcast through antennas. These last components also add more noise to that already present in the video signal.
The noise can be reduced by using different filtering methods which are chosen on the basis of the characteristics of the noise to be filtered. For example, a median filter for impulsive noise or a recursive filter for Gaussian noise give good results with an acceptable cost/performance relationship.
On pages 257-276 of the text "Digital filtering of television signals" are discussed some of the techniques presently most used for reducing video signal noise. Although advantageous in some ways, these techniques do not yet achieve good filtering by means of adaptive procedures.
In addition, as already mentioned, the other problem limiting the good quality of video signals is the reduced amplitude of the signal band. It is recalled that the television signal is a three-dimensional signal having two spatial dimensions, one horizontal and the other vertical, and a third temporal dimension which allows reconstruction of the movement of the scene taken and then reproduction of moving images. We shall define below as an image field an assembly of rows which make up a television image in a given time period. The problem of limited band amplitude must be considered with reference to the three dimensions of the signal.
It is known that an improvement in the spatial resolution of the video image can be achieved with spatial filtering which adds a peaked signal to the original signal. An improvement in the time resolution is achieved through interpolation algorithms. In any case, the circuitry architectures which allow video signal resolution improvement must perform a spatial or space-temporal filtering of the signal.
The filtering performed allowing only for the spatial information is executed with interpolation algorithms called `intrafield` because they make use of the information present in the same image field. The algorithms which make use of the information present in two consecutive image fields detecting the presence of movement (space-time filtering) are termed `interfield`.
Temporal resolution can be increased, even doubling the image field frequency, by means of a Scanning Rate Converter (SCR). As an alternative the temporal resolution can be increased by doubling the line frequency of an image field.
But in all cases, whether noise filtering or video image filtering, low-pass filters are normally used. In this manner, however, the contours of the objects in the image are shaded or attenuated and it becomes necessary to use additional and costly directional filters which supply a remedy for this shortcoming. The low-pass recursive filters also attenuate the high temporal frequencies correlated with the movement content of the image sequences. These filters should thus have a behavior such that when a movement is detected no filtering is done.
In these cases it would be desirable to be able to associate with the filter architecture a movement detector.
Indeed, if we consider the pixel belonging to a static region of the video image it can be reproduced without filtering so as to safeguard the display resolution. But if the pixel belongs to a moving image region it is the interpolated value of said pixel which will have to be displayed.